American forces have launched twin raids against Islamist militants in Libya and Somalia, capturing a long-sought Al Qaeda leader wanted over the 1998 bombings of two US embassies.

Abu Anas al-Libi has been on the run for 15 years and has already been indicted in a US court for the bombings of the embassies in Kenya and Tanzania.

The bombings left more than 200 people dead.

Pentagon spokesman George Little says he was captured during a broad daylight operation in the Libyan capital Tripoli.

"As the result of a US counter-terrorism operation, Abu Anas al-Libi is currently lawfully detained by the US military in a secure location outside of Libya," he said in a statement.

His capture paves the way for Libi, 49, to be taken to the US to face trial.

In Tripoli, a source close to Libi has told AFP he had been "kidnapped close to his home after dawn prayers by a group of armed men".

Libyan security services say they are unaware of any kidnapping or arrest of the man.

The US government had put a reward of up to $US5 million on Libi, who is on the FBI's Most Wanted Terrorists list and was born under the name Nazih Abdul Hamed Al-Raghie.

According to the indictment, Libi and other Al Qaeda members discussed an attack on the US embassy in Nairobi as early as 1993.

He is said to have conducted visual and photographic surveillance of the mission that year.

In 1994, he allegedly planned to attack the mission as well as the building, then housing the United States Agency for International Development in the Kenyan capital, along with British, French and Israeli targets.